Source-sink manipulations suggest an N-feedback mechanism for the drop in N2 fixation during pod-filling in pea and broad bean

Various legume species show a marked decline in N2 fixation during pod-filling. The objective of this study was to clarify whether this is a result of impaired nodule assimilate supply or whether re-moblised N from senescing lower leaves initiates the phenomenon through an N-feedback impact. In model experiments on pea and broad bean plants during vegetative and reproductive growth, 30 or 60% of green leaves were either excised or individually darkened, thus removing the same photosynthetic capacity yet allowing N to be re-mobilised from darkened leaves. Results are consistent with an N-feedback down-regulation of nitrogenase in that 1. leaf darkening reduced N2 fixation to a greater extent then excision, 2. darkened leaves quickly senesced and N from these leaves was re-mobilised in substantial amounts, 3. N and amino acids (AA) accumulated and C/N ratios decreased in nodules of plants with darkened leaves versus excision or untreated controls. These findings further support various indirect evidence that nitrogenase is regulated by an N-feedback mechanism that is not yet fully understood.